A drawback with light valve projectors is that a certain amount of light leaks through in the black state i.e. a full black image is not black, but very dark grey representing a limitation in the contrast ratio. This is particularly problematic in display systems that make use of at least two projectors to constitute an image. An example is given in FIG. 1. In FIG. 1 the left hand projector proj 1 is shown back projecting a first image onto half of a display screen. The second half of the image is projected by a right hand projector.
A first approach is to stitch two images to one single image but due to projector misalignments and projection lens errors this will always result in a disturbing dark or bright seam in the image, shown schematically FIG. 2—front view showing black or discoloured region between the two images. In FIG. 2 a first projector (proj 1) projects half of the image onto a projection screen and a second projector (proj 2) projects the other half. An attempt is made to align the two images exactly so there is no overlap and no gap. An alternative approach is to arrange the projectors such that their images will partially overlap. However, without corrective actions this will lead to zones in the image where the brightness basically is double, see FIG. 3. One could consider reducing the brightness in the overlap zone e.g. electronic compensation is one method to try and correct such problems. However, for dark images this adjustment is not possible, since the pixels are already at their minimum value all over the image. Therefore in the overlap region of dark images the brightness will be double (FIG. 4). This double black artefact is inherent to electronic blending and the only way to obtain a uniform black state is by increasing the black level outside the overlap region. However doubling the brightness of a dark state implies halving the contrast ratio of the image.
Another approach is to use a kind of binary mask somewhere near an image plane, for instance between projection lens and image (FIG. 5). FIG. 5 shows one of a plurality of projectors with the image projected through a binary optical mask. The mask either passes or blocks the light. A simple metallic plate can be used for this. The transition zone between bright image and dark image is determined by the size, position and filling of the lens aperture and the position of the hard filter with respect to this lens aperture. This brightness transition is typically non linear and there is only limited adjustment possibilities to control the shape and width of the blended area. Therefore this technique will require almost always an electronic clean up method to reduce the brightness artefacts. Nevertheless in some simple display configurations it may lead to acceptable results. For more challenging display systems such as a flight simulator, where curved screens and a multiple of projectors are used, this technique can hardly result in a high quality image.
A still further approach is to use an optical filter with multiple grey scales (FIG. 6). In this case optical blend filters are used between each projector proj 1 and 2 which have a range of grey scales. A problem with this is that grey scales are achieved by dither patterns if they are printed. The denser the dither pattern, the darker the grey level (FIG. 7). These dither patterns consist of dots—the more dots the darker the image. Inserting an optical filter with a dither pattern in the light path between light valve of a projector and image gives rise to unacceptable artifacts such as diffraction or visibility of the dither pattern in the image. The higher the dither pattern resolution, the higher the diffraction becomes. Diffraction can then in turn be minimized using a randomized dither pattern. However a randomized pattern will spread the incoming light over all possible diffraction orders, i.e. it will scatter light in all directions which results in a poor image ANSI contrast. Besides this the sharpness, i.e. the modulation depth (or modulation transfer function, MTF) of the image will get worse.
U.S. Pat. No. 5,077,154 discloses a soft edge mask for use in the projection of a photographic image on to a viewing screen and comprising: a panel; a continuous mask portion formed by a substantially opaque area of said panel and defined by a clearly focussed edge of a first size and having a predetermined rectangular, triangular, square or star shape; a continuous clear portion formed by a substantially transparent portion of said panel and defined by a clearly focussed edge of the same said predetermined shape but having a second size different from said first size; and a margin portion extending between said mask portion and said clear portion, said margin portion having a light transmissability progressively decreasing from said transparent portion to said mask portion and, at all positions between said transparent portion and said mask portion, being clearly focussed and having a shape the same as said predetermined shape but being proportionately sized relative to said first and second sizes and intermediate said first and second sizes, whereby, when said soft edge mask is used in projecting a photographic image, a first portion of such a photographic image is projected on to such a screen, a second portion of such an image is masked from such a screen, and a marginal portion of such an image between said first and second portions is progressively faded without significant loss of resolution and without significant distortion of said predetermined shape. U.S. Pat No. 5,077,154 further discloses that the margin portion preferably comprises a plurality of strips, each being defined by edges of the same said predetermined shape and said strips being of progressively varying light transmissabilities and that these masks may be used in front of a first (slide) projector so as to permit a second (slide) projector to blend its image into the first one.
Conventional methods can be very tedious and labour-intensive for making soft edge masks. A first technique to apply these inks is by screen printing. The inks are not liquid inks but are pastes. Two important drawbacks are that every grey level requires another screen print mask (FIG. 10) and it is difficult to control the layer thickness of the inks. Equation (1) below shows that thickness variations will give rise to transmission variations:I=I0·exp(−k·d)in which I0 is the light intensity at the entrance of the layer (after surface reflection), I is the light intensity at the exit of the layer (before surface reflection), k is the absorption constant and d is the thickness of the layer. The absorption constant k depends on the ink that is considered. A second technique is conventional black and white photography on glass plates using digital exposure techniques. Conventional photographic films are unsuitable due to the inability of the gelatine binder to withstand the high temperatures and high light intensities without degradation.
There are generally two important drawbacks of known printing techniques, namely the are the limited maximum optical density of the current dyes and the limited resistance of the current dyes to the high light intensities involved. In addition it is not straightforward to print directly on glass.